Sometimes they're absolutely right and contain key information, but the rule is that they're not to be trusted. Not only are previews, by nature, a first impression - they're also part of a now very large business with many strong interests to be taken care of.

Most AAA titles live or die by their hype and first impressions, so naturally the first hours of gameplay will be the strongest. Bethesda is a very good example, because their games tend to make a very strong first impression - even on seasoned gamers. Previews in this case will be nearly 100% positive - and this is without the corruption factor.

In effect, I simply don't use previews for anything except a hollow passtime. Sometimes they contain interesting factual information, but even that is something to be sceptical of.